Well the morning winds finally dropped below 15kts and the chop in the marina subsided and I finally got to practice with my new ATN Mastclimber. Thanks for your suggestion on how to rig the safety line. I attached the jib halyard to a stancion base and winched it down as tight as I could get it and attached the ATN to it. Then I attached the main halyard to a padeye on the mast and cleated it off as tightly as possible and put a safety strap using double fisherman knots to make a loop and a prussic knot to attach to the main halyard. Thanks for your suggestion, worked like a charm.
Denise expressed having some difficulty in using the ATN in another thread and a couple of SNers recommended small steps. Good advice. Going slow with small steps and remembering the sequence: stand, raise chair, raise safety loop, sit, raise feet, stand....
This, however is not to say the whole operation is like eating cake! First of all that tiny skinny old halyard no longer looks so robust. I know it's 3/8 Stayset X and is only 3 yrs old, but....
Everything feels spungy, like you are on a trampoline or bungy jumping, but when you look closely you are just swinging back and forth, not up and down.
Turns out the hardest part is the toll it takes on your arm muscles. I don't normally use my arms like that so I fatigued rather easily. But, slow small steps with rest stops helps.
So I got the shroud down, will fabricate the replacements today and install them tomorrow.
Thanks to SailNet Community for good advice and information.